Game balance
Game balance is a general concept in roleplaying games. It is an ideal of game-design - in a balanced game, all options for character growth and generation are "balanced" in the sense that they have equal impact on the game, leaving only roleplaying considerations as the basis on which to choose. Thus, two equally-advanced characters will be equally capable. The players would be free to roleplay without feeling that their "suboptimal" choices made their character irrelevant and impotent.
Game balance is an impossible ideal to achieve. One reason is that different groups put emphasis on different aspects of the game, and different options will affect their games differently; for example, in a group that places great emphasis on social interaction rolls, the Social Handicap flaw might be a Major one, while for a group that plays only very quick sagas, that never cover more than a few (in-game) years, the Age Slowly flaw will be irrelevant. In practice, game balance as a concept is often restricted to combat effectiveness, and to "normal" sagas.
Very balanced games also make for boring games, as all options are essentially the same.
Ars Magica is unlike many other games in that it explicitly rejects game balance to a large degree. Instead, Ars Magica tends to allow characters to specialize in certain roles, and promotes a more organic and less balanced character growth. Another reason to abandon game balance is that different characters in Ars Magica can be "advanced" to different degrees (having different levels of experience and age). Game balance is still important to some degree, however.
The most glaring violation of game balance is the division into dramatic roles: minor, background characters (grogs), supporting cast (companions), and major protagonists (magi). The background characters usually are very weak, with companions usually being more effective but still no match for a capable magus in most areas. The great exception are social interactions with mundanes, where the roles are often reversed - companions often being capable, and magi (often) being incapable. Companions can also excel at a limited area, such as some Supernatural Ability, and grogs typically are more capable at Defense than magi, but still overall far weaker in combat. Magi typically need companions to help them socially and grogs to isolate them from serious harm in combat.
The Mythic Companion is apparently intended to serve as a major character that is not a magus, and be of comparable might. In practice, most Mythic Companions are more limited in their power and especially in their advancement options compared to magi, and will probably be weaker in a long-running saga.
Even between magi there are power disparities. Generally, not all options are equal and some magi will be less effective; a Creo Ignem specialist will probably be quite capable in combat, whereas an Intellego specialist will probably be less effective. The different arts are not balanced against each other for combat effectiveness, and it is clearly intended (at least) for some combinations to more generally more effective. Likewise, some Houses and character concepts (like a pacifistic Criamon) are obviously less effective than others (like the Flambeau hoplite). How effective the character will be in the saga is another matter.
Different virtues and flaws are more-or-less balanced against each other (within their categories - a Major Virtue compared to other Major Virtues, a Minor Flaw compared to other Minor Flaws, and so on). Some are still considered more or less effective, of course, although (as always) actual usefulness depends on the saga and character. For example, Puissant Art provides a larger bonus, and is therefore more effective, than Affinity with Art for sufficiently small Art scores - so Puissant Art is more powerful for short sagas, where the character isn't expected to have opportunity to gain experience past the tipping point. Note that in-game they can represent very different things (a child prodigy will have Puissant, while a child fated to be a great master of the Art will have Affinity), and the player's choice can be based on such roleplaying considerations, not just brute force.
Game balance is also important in that some choices may "break" the assumed balance of the game, even if it is not the ideal equal balance. Options that seem to be extremely effectively yet easy to achieve, or not as effective as the difficulty and nature of the effort seems to imply, are considered "unbalanced". For example, relatively low-level rituals can create enough wheat to feed entire kingdoms. Groups that feel that this can undermine the setting may choose to add ad-hoc magnitudes to such spells, to preserve the "balance" of the game. The over-effective spell "breaks" the game, and needs to be corrected.